Impossible to append from a library implementing composer-scaffold

Formatting

When I tried to replicate this issue on a pure core 10.3 , unfortunately I don't have access to PHP 8.3 to testing on Drupal 11, the behaviour was different - requiring main-settings library scaffolds settings.php as expected, however requiring custom-settings library scaffolds the settings-custom.php and completely ignores the main settings.php - there is even no mention of it in the output related to the custom-settings library ... Feels like that file is completely ignored.

However if I remove main-settings library and leave settings.php in place - custom-settings library appends to it as expected:

Scaffolding files for library/custom-settings:
  - NOTICE Modifying existing file at [web-root]/sites/default/settings.php. Examine the contents and ensure that it came out correctly.
  - Append to [web-root]/sites/default/settings.php from assets/settings-php-include.txt

I've already was burnt by the order once and somehow managed to forgot about it - order of the packages in the `allowed-packages` list is very important!

My libraries was ordered incorrectly in the top level composer.json:

"drupal-scaffold": {
            "allowed-packages": [
                "library/custom-settings",
                "library/main-settings"
            ],
        }

and it should be

"drupal-scaffold": {
            "allowed-packages": [
                "library/main-settings",
                "library/custom-settings"
            ],
        }

.

All of this is properly documented in the README.md "Allowed packages" section.